This invention relates to a driving force distribution system in a hybrid vehicle of the type in which the front or rear set of wheels is driven by an engine and the other set of wheels is driven by a motor. The distribution system is adapted to changeover the distribution of driving force based on the amount of charge remaining in a battery.
There have been various proposals relating to four-wheel drive hybrid vehicles in which the front or rear set of wheels is driven by an engine and the other set of wheels is driven by motors. The applicant has also made a number of such proposals. According to one such proposal (see Japanese Patent Application No. 62-37183), the distribution of driving force to the front and rear wheels is performed based on accelerator opening, vehicle velocity and shift lever position, and the distributed values are revised based on the amount of battery charge. The proposal as disclosed in the previous application will now be described.
FIG. 1 is a view illustrating an example of the control system configuration of a hybrid vehicle, FIG. 2 is a block diagram showing an example of the construction of a control circuit, and FIG. 3 is a view illustrating the relationship between the amount of battery charge and various detected values. Shown are an engine 11, controllers 12 and 16, a battery 13, a detector 14 for detecting the amount of battery charging (the amount of remaining battery life), a computer 15 for control, motor 17 and 18, an input interface 21, a CPU 22, a ROM 23, a RAM 24, and an output interface 25.
As shown in FIG. 1, the engine 11 drives the front wheels of the vehicle and the motors 17, 18 the rear wheels of the vehicle. The battery 13 is used as the power supply for the motors 17, 18. The detector 14 detects the amount of charge in the battery 13 based on the battery voltage or current, the concentration of the battery electrolyte or the specific gravity of the electrolyte. The output signal of the detector 14 is applied to the control computer 15. As shown in FIG. 2, the control computer 15 includes the CPU 22, the ROM 23, the RAM 24, the input interface 21 and the output interface 25 and is adapted to set vehicle driving force based on accelerator opening, vehicle velocity, shift lever position and the like, as well as the front and rear wheel torque distribution values based on load where each of the front and rear wheels contact the ground, this in turn being based upon the load distribution of the vehicle. The torque distribution values are outputted to the controllers 12, 16 upon correcting the torque distribution values in dependence the amount of charge in such a manner that the value of torque distributed to the motors 17, 18 is decreased and the value of torque distributed to the engine 11 increased correspondingly if the amount of battery charge is small. The engine controller 12 controls engine torque by controlling throttle opening or the amount of fuel injection in accordance with the torque distribution value. The motor controller 16 controls motor torque by controlling the current which flows through the motors in accordance with the torque distribution value.
The specific gravity and concentration of the electrolyte and the battery voltage vary depending upon the amount of charge, as shown in FIG. 3. As in conventional vehicles, means for ascertaining the amount of battery charge include an ammeter for detecting charging/discharging current, a voltmeter for detecting battery voltage, and a battery charge lamp which provides a warning indication when battery voltage falls below a reference value, The driver operates the vehicle while determining the amount of battery charge by observing these instruments. The detector 14 detects the amount of battery charge from battery voltage or current or from the concentration or specific gravity of the electrolyte basically in the same manner as the abovementioned instruments.
With this hybrid vehicle driving force distribution system arranged as described above, motor and engine outputs are adjusted by the control computer 15 so as to lower motor output and reduce power consumption when it is determined that there is a small amount of charge in battery 13. In other words, control is such that motor driving force is reduced by decreasing motor output while engine output is raised correspondingly. The end result is that overall driving force of the vehicle does not change.
However, with this hybrid vehicle driving force distribution system, the amount of battery charge can be determined in terms of only three stages, namely high, medium and low. A problem which arises as a result of this is that when the amount of charge changes, driving force distribution suddenly changes and therefore the vehicle cannot travel in a stable manner.
Another problem is that since there is no charging mode, the battery cannot be charged automatically when there is a decrease in the amount of battery charge. As a result, nothing can be done to check consumption of the battery.